268 
MOUNTAIN SLIDES. 
tliat to which the destructive effects of mountain torrents are 
chiefly due—the felling of the w r oods. 
In nearly every case of this sort the circumstances of which 
are known, the immediate cause of the slip lias been, either an 
earthquake, the imbibition of water in large quantities by bare 
earth, or its introduction between or beneath solid strata. If 
water insinuates itself between the strata, it creates a sliding 
surface, or it may, by its expansion in freezing, separate beds 
of rock, which had been nearly continuous before, widely 
enough to allow the gravitation of the superincumbent mass 
to overcome the resistance afforded by inequalities of face and 
by friction ; if it finds its way beneath hard earth or rock 
reposing on clay or other bedding of similar properties, it con¬ 
verts the supporting layer into a semi-fluid mud, which opposes 
no obstacle to the sliding of the strata above. 
The upper part of the mountain which buried Goldau was 
composed of a hard but brittle conglomerate, called nagelflue , 
resting on an unctuous clay, and inclining rapidly toward the 
village. Much earth remained upon the rock, in irregular 
masses, but the woods had been felled, and the water had free 
access to the surface, and to the crevices which sun and frost 
had already produced in the rock, and of course, to the slimy 
stratum beneath. The whole summer of 1806 had been very 
wet, and an almost incessant deluge of rain had fallen the day 
preceding the catastrophe, as well as on that of its occurrence. 
All conditions then, were favorable to the sliding of the rock, 
and, in obedience to the laws of gravitation, it precipitated itself 
into the valley as soon as its adhesion to the earth beneath it 
was destroyed by the conversion of the latter into a viscous 
paste. The mass that fell measured between two and a half 
and three miles in length by one thousand feet in width, and 
its average thickness is thought to have been about a hundred 
feet. The highest portion of the mountain was more than 
three thousand feet above the village, and the momentum 
acquired by the rocks and earth in their descent carried huge 
blocks of stone far up the opposite slope of the Rigi. 
The Piz, which fell into the Cordevole, rested on a steeply 
